Method of producing transparent polystyrene sheets



J. OQBEATTIE Jan. 12, 1954 METHOD OF PRODUCING TRANSPARENT POLYSTYRENE SHEETS Filed Aug. l, 1951 Patented Jan. l2, 1954 n METHOD OF PRODUCING TRANSPARENT l POLYSTYRENE SHEETS John 0. Beattie, Stamford, Conn., assignor to Cast Optics Corporation, Riverside, Conn., a

corporation of New York Application August l, 195'1-,Serial No. 239,792

This invention relates to the formation of sheets of a polymerized monomer. Moreparticularly the invention relates to making such sheets in optically clear condition. An object vof this invention is to devise ways and means for making such sheets of polystyrene which is a rather inexpensive starting material. Previous attempts to accomplish this object have encountered dimculties in that such sheets have not been satisfactory optically; they have had voids or bubbles in them; they have been too brittle; they have been cloudy or hazy; they have not been water-white, but yellow; and they have had crazing on their surface. So it is another object of this invention to produce sheets of polymerized styrene completely free of these disadvantages. And another object is to discover how to exercise close control of the polymerization of the styrene at all times in order to avoid the occurrence of these characteristics, each and all of which degrade the resulting sheet if not rendering it generally non-usable. The discovery of how to exercise and realize this control has been diiicultV of attainment because there seems to be so little previously known of just what happens during the polymerization of styrene and of the mechanisms thereof. So another object is to lind out what happens and how to control it. Yet another object is to make use of the styrene in its cheapest and most abundant form as now marketed.

Styrene is a vinyl benzene resin and never gels in the pure form as now marketed. It has only one active radical in its molecule and consequently polymerizes to form linear chains instead of a three-dimensional network. As it nevergels, it must be treated differently from all those monomers that become gelated during polymerization. It merely progressively increases its viscosity or consistency during polymerization and its mold-curing temperatures are such that this thickening takes place while the styrene is in liquid-phase or at least the viscousphase so the mold is not to be opened until a substantially solid-phase degree of cure has been reached, which may be followed by an annealing stage. Undoubtedly, the voidor bubble-formation, the formation of haze, and the tendency to craze are related to the conditions under which this thickening type of polymerization takes place, so it is another object of this invention to find out just what these conditions are and Vthen avoid such tendencies to produce these `degrading results.

'of the watery styrene.

cured within the mold, so it is another object of this invention to nd out how to thicken the styrene as much as economically possible before it is put into the mold.

Another object was to try to nd a catalytic reagent that is usable with the styrene during its polymerization that not only would facilitate the latter, but by the use of which there could be controlled -a predetermined higher average molecular weight of the polystyrene to produce a sheet thereof with the desired toughnessas opposed to brittleness produced by lower molecular weights. Lower molecular weight are produced to some extent by higher curing temperatures, so it is another object to cure the styrene at the lowest economical polymerizing temperatures. Foreign particles, such as dust, tend to produce bubbles and crazing which degrade the appearance of the sheet, so it is a further object to minimize theirY presence as far as possible. Y

These objects are attainable by the practice of a process that may be described generally as starting with a watery-thin styrene to which a parting agent has been added; properly thickening it; mixing with the thickened styrene a special catalytic reagent; inserting the mixture into a dustless and lintless mold formed of two glass plates clamped together with an accurately dimensioned gasket between the glass plates adjacent their edges; inserting in an oven such glass mold containing -the styrene mixture and subjecting the mold to heating carefully controlled as to time and to degree so that the mold is `subjected to a low heat stage, a middle heat stage, and a high heat stage, followed by an annealing stage; whereupon the mold is opened, and the formed sheetof polystyrene peeled from one of the glass plates.

In the drawings:

Figure l is a diagrammatic showing of the process; and

Figure 2 is another diagrammatic showing of the process including details of the several steps.

In greater particularity, the parting agent is stearic acid used in quantity of 1% by weight The proper thickening is done by boiling the mixture at 146 C. for about ten minutes under conditions that any resulting vapors are quickly condensed and the depth of the watery styrene boiled is shallow enough to permit rapid dissemination of the evolved heat. This vthickening results in a contraction l.of the'mass byrabout 3.4%. Thereafter, the thickened mass is cooled to room temperature and then is mixed with from 1A, to l@ per cent of isopropyl percarbonate, and inserted into the mold of glass plates. The sandwich of two such plates with the fluid thickened half an hour during which 'the. cured polymerized styrene sheet passes from viscous liquid-phase to solid-phase, or substantially so. While the mold is at '73 C. or within not more than Y2"C. thereunder, the mold is placed horizontally vand is opened by wedging one plate of glass from the other and removed, leaving the .sheet of .polystyrene exposed, whereupon that sheet 'ispeeled from 'the otherplate of glass that is supporting it,V and `then stacked afterpreferably 4spray-coatingit with d protective flexible coating'such as polyvinyl alcohol, or. low '.tack masking paper, for Vproducing a peelablaprotectng'coating on both sides of the polystyrene's'heet. .Thesheet continues .to cool andto shrink, but the shrinkage takesjpla'ce uniformly. andfreely.

The catalytic reagent is uniqueLin that it functions at-the relatively low 'temperatures used, and unlike benzoylperoxide itgivesno yellow cast especially to the-edge of the sheet, orany hazy orcloudy areas. Benzoyl peroxide requires Cil ' of the styrene and bubbles will result.

a temperature of cure around'25 5C. 'higher than'isopropyl *.percarbonate. Aisopropyl percarbonate seemsto act Aas. a chemical reagent as well as acatalyst because it becomes all used up,

incorporating itsel into the,,polyniertchairry and A cannot' be identied in thenal product. That is why it is referredto herein as a catalytic reagent. .Moreovenl it offers adenite control of the physicalcharacteristics ofithepolystyrenesheet, name- 1y, whether .it .is brittle or tough. 'This particularlcatalytic reagent permits, polymerization to occur. at relatively' low'. temperature, hence it minimizesformation of bubbles;fit helps to minimize..adhesion V,of the. fully cured .resin to ithe mold .and thereforejmproves parting; .andv its use with.. styrene seemsto require lessitime of .curing.in-. the mold.

'j .The molecular weight and. hence thephysical `and.thernlal.properties of the polystyrene depend very strongly. upon theV amountof catalystused. kIf asI much as 1% .ofisopropyl percarbonateis used, the resulting` sheet Will be extremely brittle and will shatter almost as readily as glass. If 1/4 of 1% catalyst is used, the resulting sheetgis very tough and has properties.ye1y..closelyap .preaching ,that of..commerci'elly .molde'dp'olystyrene. Using 1/4 of l% catalyst with 'the cur- -ing cycledescribed herein, the molecular Yweight of `the kpolystyrene is 'probably around 120,000. .I .have found in practicethatit is .not advisableto go ,aboveY 1A) of 1% catalyst,A .in order.. to preserve the -desirable .physical properties, ofthe sheet. Going muchibelow %.of .1% catalyst causesidif- :Iiculty .in curingrwithl formation .of Abubbles very likely.. to .occur.

Asto thetemperature rangesgivemin the lowheat stagev it.may`.be varied somewhat depending upon the .thicknessrofgthe sheet.. being cured. Fon-sheets of fromgg" 't0 1/f."v 'the temperature f-this, stage cango to 55C. Inggenefal'rthicker sheets require lower temperatures here, while thinner sheets require higher temperatures. In this low-heat stage, if the heat goes above 55 C. the heat of polymerization is given oi faster than it can be taken away by the oven, so that general temperature will rise to the boiling temperature If this low-stage heating takes place at less than 35 C. the required time is excessive and what is Worse, the styrene solidies to become extremely viscous so that polymerization shrinkage occurs faster -thantheviscous material can iiow so this shrinkage pulls voids (bubbles) in the sheet. In the .middle-heat stage,if the temperature rises too high, the catalytic reagent is used up inefficientv.lybecause the higher temperature decomposes it and this prevents proper polymerization of the styrene.

As to-the temperature to which the sheet is cooled after annealing, Vbefore the mold is opened, ,the area of the sheet has to be considered, because the larger the sheet, the greater is the overall shrinkage. A sheet of say2' x` 2 .or less, `icanbe `opened at a much cooler temperature, Ieven at `50" C., provided proper Yannealing `has 'beengiven it. At 75 C. the resin is in la viscous fluid state, whereas at 71 C. it is in a solid state, so '73" C. has beenpicked as a safe middle temperature `at which to open the mold. Atrany rate, theannealingstage is important because if 'cooled too rapidly,pthestrains of the shrinkage freeze themselves into the polystyrene unless sev- `erai hoursrof annealing time are provided to give the strains time to relieve and equalize themselves, Aotlfienvise thesheet will craze. `And this erazing effect v/illY be increased, if the cooling takes place outgof the oven.

A sheet cured with 1/4% catalyst along the cycledesciibed above, was found to have the following properties:

. Property Results Volatile, percent... 3. A58 Viscosity, cps 19. 5 Impact strength:

Notched, it'. lbs/in. of notch .n 0. 34 Unnotehcmft. lbs/in. of Width 1.26 YMolecular Weight, (light scattering method) 143,000

" What -isfclaime'd is:

'1.' The process of producing crazeV and bubble free molded transparent sheets of polymerized fstyrene'comprising 'the steps o1" adding a parting agent" to styrenerof watery thin consistency, thickening-the mix-ture by boiling at a temperature of aboutf146"Cffor a period ofabcut 10 minutes,- aliowing the mixture' to co'ol to -room temperature, carrying out the heat polymerizationfoi the thickened mixture by admixing therevwith'isopropylpercarbonate in the amount of from .125% to 15% by weight of 'the 'thickened .-n'fiixture, placing the'mixture in a sheet'forming '.nx'old,A curing 'the-'mixture in the mold by subject- "ing the mold to low, intermediate, and high temperature Istages'for preselected periods of time, Where in Vthe' low temperature stage the mold is maintained at a temperature not exceeding 55 C; and not less than' 35 C. depending on the thickness of the mixture in the mold, for 15 hours, Where in the intermediate stage this mold is heated from the temperature of the first stage to aV temperature of 80 C. over a. period of l2 hours, and where inrtherhigh stage the mold-is maintained at atemperature of C.-for about 6'.hours," thereafter reducingA the temperature of the mixture within the mold to an annealing temperature of 73 C. and maintaining said temperature for about 30 minutes and removing the mixture from the mold at a temperature of from '71 C. to '73 C.

2. The process of producing craze and bubble free molded transparent sheets of polymerized styrene :comprising the steps of adding stearic acid to styrene of watery thin consistency, thickening the mixture by boiling at a temperature of about 146 C. for a period of about 10 minutes, allowing the mixture to cool to room temperature, carrying out the heat polymerization of the thickened mixture by admixing there- With isopropyl percarbonate in the amount of from .125% to .5% by weight of the thickened mixture, placing the mixture in a sheet forming mold, curing the mixture in the mold by subjecting the mold to loW, intermediate, and high temperature stages for preselected periods off time, where in the low temperature stage the mold is maintained at a temperature not exceeding 55 C. and not less than 35 C. depending on the thickness of the mixture in the mold, for hours, where in the intermediate stage this mold is heated from the temperature of the rst stage to a temperature of 80 C. over a period of 12 hours, and where in the high stage the mold is maintained at a temperature of 80 C. for about 6 hours, thereafter reducing the temperature of the mixture Within the mold to an annealing temperature of '73 C. and maintaining said temperature for about 30 minutes and removing the mixture from the mold at a temperature of from '71 C. to '73 C.

3. The process olf producing craze and bubble free molded transparent sheets of polymerized styrene comprising the steps of adding a parting agent to styrene of watery thin consistency,

thickening the mixture by boiling at a temperature of about 146 C. for a period of about 10 minutes, allowing the mixture to cool to room temperature, carrying out the heat polymerization of the thickened mixture by admixing therewith isopropyl percarbonate in the amount of from .125% to .5% by weight of the thickened mixture, placing the mixture in a sheet forming mold, curing the mixture in the mold by subjecting the mold to low, intermediate, and high temperature stages for preselected periods of time, Where in the 10W temperature stage the mold is maintaind at a temperature not exceeding C. and not less than 35 C. depending on the thickness of the mixture in the mold, for 15 hours, Where in the intermediate stage this mold is heated .from the temperature of the first stage to a temperature of C. over a period of 12 hours, and Where in the high stage the mold is maintained at a temperature of 80 C. for about 6 hours, thereafter reducing the temperature of the mixture Within the mold to an annealing temperature of '73 C. and maintaining said temperature for about 30 minutes and removing the mixture from the mold at a temperature of from '71 C. to '73 C., and finally spray coating the sheets with a protective stripable flexible coating.

JOHN O. BEAITIE.

References Cited in the file of `this patent UNITED STATES PATENTS Number Name Date 2,256,160 Britton et al. Sept. 16, 1941 2,357,833 Kropscott et al. Sept. 12, 1944 2,385,486 Bartoe et al. Sept. 25, 1945 2,559,166 Morner et al. July 3, 1951 

1. THE PROCESS OF PRODUCING CRAZE AND BUBBLE FREE MOLDED TRANSPARENT SHEETS OF POLYMERIZED STYRENE COMPRISING THE STEPS OF ADDING A PARTING AGENT TO STYRENE OF WATERY THIN CONSISTENCY, THICKENING THE MIXTURE BY BOILING AT A TEMPERATURE OF ABOUT 146* C. FOR A PERIOD OF ABOUT 10 MINUTES, ALLOWING THE MIXTURE TO COOL TO ROOM TEMPERATURE, CARRYING OUT THE HEAT POLYMERIZATION OF THE THICKENED MIXTURE BY ADMIXING THEREWITH ISOPROPYL PERCARBONATE IN THE AMOUNT OF FROM .125% TO 5% BY WEIGHT OF THE THICKENED MIXTURE, PLACING THE MIXTURE IN A SHEET FORMING MOLD, CURING THE MIXTURE IN THE MOLD BY SUBJECTING THE MOLD TO LOW, INTERMEDIATE, AND HIGH TEMPERATURE STAGES FOR PRESELECTED PERIODS OF TIME, WHERE IN THE LOW TEMPERATURES STAGE THE MOLD IS MAINTAINED AT A TEMPERATURE NOT EXCEEDING 55* C. AND NOT LESS THAN 35* C. DEPENDING ON THE THICKNESS OF THE MIXTURE IN THE MOLD, FOR 15 HOURS, WHERE IN THE INTERMEDIATE STAGE THIS MOLD IS HEATED FROM THE TEMPERATURE OF THE FIRST STAGE TO A TEMPERATURE OF 80* C. OVER A PERIOD OF 12 HOURS, AND WHERE IN THE HIGH STAGE THE MOLD IS MAINTAINED AT A TEMPERATURE OF 80* C. FOR ABOUT
 6. HOURS, THEREAFTER REDUCING THE TEMPERATURE OF THE MIXTURE WITHIN THE MOLD TO AN ANNEALING TEMPERATURE OF 73* C. AND MAINTAINING SAID TEMPERATURE FOR ABOUT 30 MINUTES AND REMOVING THE MIXTURE FROM THE MOLD AT A TEMPERATURE OF FROM 71* C. TO 73* C. 